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Full text of "The basic physics of fission and fusion weapons"

JOURNAL 



of the 



U.B.C. Physics Society 



~S/////////*f, ,,,.,, .,,,,, 



4- 
3- 



3 



Paschen 



Brackett 
series 
n'-4 



n'-3 



Sal mer series 
n'-2 



Lyman series 
n' - 1 




Quantum formula for tha anargy of an alactron bound in a 
hydrogen atom 

_ I me* * 

n » principal quantum number with possible values 1, 2, 3, . . . 

e — electron charge 
m * electron mass 
Co ■ permittivity of free space 

h m Planck's constant 



VOLUME 25 



tlTfjc ©ntbtrsttp of 
Pritistf) Columbia 



NUMBER 1 



-£a£ 



im%»l»##»» 



1986 



TABLE OF CONTENTS 

Presidential Message i 

Vice-Presidential Message .. . ii 

Faculty and Journal Editor Messages iii 

Information for Contributors iv 

Graduate Study Advertisement v 



R. I. Thompson 

T. Sato 

G. M. Hill 

J. R. de Bruyn 

C. Chan 



P. Bruskiewich 

and 
G. Felton 



Rotations in Special Relativity .„... 1 

Calibration of Millimetre- 
wavelength Spectra: CO Observation 
with the NRAO 12m Telescope 14 

The M -W(H ) Relation for 0, B, and 
A Supergiants Recalibrated with High 
Signal-to-Noise Reticon Spectra ... 29 

The Statistics of LOTO 6/49 62 

Propagation of Radiowaves in the 
r .Ionosphere . . . . . . .-.-.- . r ,. ......... 71 

The Basic Physics of Fission and 
Fusion Weapons 91 




Library of Congress t 35-805MM Journal of the Physics Society Published June. 1986 



91 



The basic Physics Of Fission and Fusion Weapons 
Patrick tiruskiewich, ti.Sc. 



A brief layman's introduction to the basic physics ol 
lission and lusion veapons is presented. All Material 
presented in this paper represents either previously 
declassified information or educated guesses on the part of 

the authors. 



In retrospect the development of fission and lusion veapons 
seems an inexorable process. Where from the turn of the 
century up until the outbreak of the Second World War the 
scientific desire to understand acted as the global impetus 
behind the basic research into nuclear physics, the 
unexpected discovery of nuclear fission by Hahn and 
Strassman at the Kaiser Wilhelm Institute on December 22nd, 
1938, set off a different chain of events ***. Their 
discovery began the Nuclear Arms Kace that continues to this 
day. Against the military and psychological background of" 
World War 11 no control could be applied to the application 
of such a discovery in the midst of a global conflict. 
The events that followed that set the East against West set 
similar constraints to the limitation of armaments. fifty 
years later, with the rise of a new International order, ve 
stand teetering at the edge of uncertainty, vith tens of 
thousand of nuclear veapons standing ready to be used at any 
time by at least five vorld powers. At the present time 
there does not seem to be a vay out of this predicament. 

The authors wish to present a succinct Introduction to the 
workings of these dreaded machines of mass destruction if 
only to remind our readers that the root cause of our 
present predicament is not the progress of technology per se 
but the more fundamental Ideological and political beliefs 
prevalent in the vorld today. Ultimately it Is not machines 
but men who make war. 

These weapons of mass destruction have time and again been 
treated as abstractions, amenable to myths and misunder- 
standing, when instead they should be treated as machines 
that reflect as much as drive the political and military 
beliefs that shape our societies. We should begin to deal 



92 



with these machineB by understanding that -the task ahead 
forces us to undertake the difficult teak of changing the 
way ve think. 

Perhaps the first step is to know a little «ore about which 
ve speak. 

The Fission Weapon 

The basic type of nuclear weapon is the fission weapon, in 

which an uncontrolled fission chain reaction is used to 

produce a large amount of energy in a short period of time, 

resulting in a powerful explosion. Where in a fraction of a 

second 1 electron volt of energy is releaaed per chemical 

reaction in a chemical explosion, about 200 million electron 

volts (200 Mev) is released per f lesion in a fission 

explosion. A fissioning of a single fissile atom such as 

Uranium-235 or Plutonium-239 is therefore 200 million times 

S more energetic per reaction than the most powerful chemical 

« reaction. A simple calculation will show then that 1 kg of 

U-235 or Pu-239 is energywise equivalent to 12,000 barrels 

of crude oil, 2,000 tonnes of thermal coal or 18 kilotonnes 

(kt) of the chemical explosive TNT. 



Fission occurs in the heavy fissile materials like Uranium 
and Plutonium while fusion (the mechanism of the the fusion 
weapon — see below) occurs in the lighter atoms like 
Hydrogen or its heavier isotopes Deuterium and Tritium. A 
fission occurs when a neutron enters the nucleus of an atom 
of one of these fissile materials, is captured by the 
nucleus, and causes the nucleus to break apart (hence to 
fission). When a fission occurs a large amount of energy is 
released (200-240 Mev), the original nucleus splits into 2 
radioactive nuclei (the fission products) and between two 
and four new high energy frse neutrons (average energy about 
2.5 Mev) are released. These new free neutrons can be used 
to produce a self-sustaining chain reaction in a body of 
fissile material if the conditions are right. A chain 
reaction will result if at least one of the neutrons 
released in each successive fission produces the fission of 
another nucleus. The remaining neutrons which do not cause 
fission are either absorbed by non-fertile atoms or escape 
the material altogether. There are on average 2.5 neutrons 
produced per fission in U-235 and an average of 3 neutrons 
per fission in Pu-239. 

An example of a representative fissioning of U-235 is given 
by 

»£, , « n ___> l« fia , 1t Kr . 2 ^ ♦ 200 Mev 

and that of Pu-239 by 

^JPu * Jn — > $Rh ♦ JJln <• 3 ^n ♦ 240 Mev" : 



93 



The distribution of fission products varies somewhat vith 
the incident neutron energy. The range of fission products 
is from bromine to dysprosium (see figure 1>. 





















r 


\ 


r 


\ 




* - 




1 






\ 




s 


I 




I 




















af 














«T< 












, 



•o ao no ito mo «o 



Figure 1- Fission Yield as a function of Atomic Mass Unit 



There exists a critical mass for U-235 and Pu-239, the 
smallest amount of the material in which a self-sustaining 
chain reaction can occur. A small lump of such material 
cannot go critical because too many neutrons escape. A 
large enough mass will retain enough of the free neutrons 
produced by one fission to go on to produce other fissions 
and other neutrons and therefore maintain the chain 
reaction. If the chain reaction is allowed to grow 
unconstrained an explosion will result because a large 
amount of energy is released in a small quantity of matter 
resulting in a high energy density. Such a critical mass 
depends on certain properties of the fissile material such 
as its shape, purity, density and physical surroundings. 

Since the rate of fission (and hence neutron production) is 
a volume eflect and the rate of neutron escape a surface 
effect, the shape or geometry of the fissile mass is an 
important property. As it happens, t.he greatest volume to 
surface ratio of a simple shape is that of a sphere and is 
equal to 1/3 of the sphere's radius (a). Hence, the best 
configuration for the least quantity of material would be a 
sphere. Another property, the purity of the fissile 
material, is important in that if non-fissile atoms are 
present free neutrons will be captured by these non-fissile 
nuclei instead of causing fission. These non-fissile nuclei 
will compete for the available free neutronB and either will 
slow the rate of growth of the chain reaction or poison it 
completely. Equally important is the density Ip) of the 



94 



f iesile material. The higher the density the shorter the 
average distance neutrons travel before causing another 
fission, and therefore the smaller the critical «ass , «». 
Finally, the last important property is the physical 
surroundings of the mass. If the fissile materiel is 
surrounded by a dense medium like natural Uranium or 
Beryllium, which reflects neutrons back into the material, 
some of the neutrons may be captured and used in the fission 
which would otherwise have escaped, thus further reducing 
the critical mass. 



A great myth is that the values of the critical mass for a 
sphere of U-235 or Pu-239 are secret* ai . This is quite 
wrong I Their values are available in the open literature as 
are enough of the other important pieces of information to 
design a functioning, albeit crude, fission weapon. 
Building an actual weapon is a far more difficult task 
requiring modern industrial know how and a large scale, 
complex industrial capability. 

The critical mass of, for example, a bare sphere of pure Pu- 
239 metal in its densest phase is about lfc kg (the size of a 
small grapefruit). If the sphere was surrounded by a 10 cm 
thick natural Uranium reflector/tamper the critical mass is 
reduced to 4.4 kg, a sphere of radius 3.6 cm (the size of an 
orange). 

There is a trick to reducing the critical mass even further. 
Using a technique known as implosion, in which conventional 
explosive lenses are used to compress a mass less than 
critical to a mass more than critical, a fission explosion 
could be achieved with less than 2 kg of Pu-239, a sphere of 
about 2.8 cm radius (smaller than a tennis ball). 

It is possible to do a montecarlo simulation'** of the 
probability of the escape of a free neutron as a function of 
the mean . free path of the neutrons .A. (see footnote 2) 
divided by the radius (a) of the sphere. The result is a 
curve resembling figure 2. 



95 




radius) . 

If we make the assumption that all the neutrons which do not 
scaet! sphere cause the «-i~« t f. « p^iS" £ 
convenient -i-Pl"^ "-"P^-^ ^^ ^^ as 
produce a simple model of a """J lt Sma t and neutron 
stated previously, ^«^ on J" J ~ U (Q) of three times the 

is^o 1 ^™ :~ s^-'iSit-i ---.ass Te 8i r g r :° : 

Oc then the sphere has reached a supercritical condition and 
an explosive chain reaction can occur. 

4-.r,i«rHnfl m sDhere of Plutonium such that the final 

^ns^is 5 It*** the P in!ti.l density. If initially p - 20 
density is 5 twee x < 100 , # However, 

SJSr the 8 ipo o'nt S 1M .5«* > *» *'** • The -phere 
is now supercritical and will chain react . explosively. 

What happens is that by i-ploding ^.S'^^^^S 
^reasea^nce^/p. ' -urn^ng S *igurf " the ratio of 
.^mean free path tc > th e sphere ------ escapee^ 

consequence the Probability that ■ * neutron8 whic h 

^ f^I 'c.ueeYflesiof.nrhence the point is reached 
k +h! dumber of neutrons produced in the chain reaction 
I r^h " .ruro/VnU released within the sphere begins 
to grow out of control, resulting in an explosion. 



96 



In the atonic bomb that destroyed Nagasaki about 8 kg of Pu 
vith a purity of 90% and in the form of tvo gold clad 
hemispheres of Plutonium metal was used. The Plutonium vas 
surrounded by a dense reflector/tamper vhich had tvo 
functions. Along vith reflecting back into the sphere some 
of the escaping neutrons, the tamper's inertia helps to hold 
together the Plutonium during the explosion to prevent the 
premature disintegration of the veapon. The longer and 
stronger the Implosion can keep the fissioning material 
above critical the higher the yield per unit mass of 
material (i.e. the efficiency — the Nagasaki bomb had an 
efficiency of 15X). 

If after Implosion an average of 2.6 neutrons are produced 
and retained vithin the mass per neutron absorbed, that is 
per generation, then in 5 kg of Pu CI. 25x10' ■ nuclei) an 
efficiency of 30% (typical of a modern veapon) vould require 
the folloving number of generations to fission: 

(2.8)- « (0. 30) (1.25xl0«« ) 

n(ln 2. 6) « In (3. 75x10* * ) 

n =55 generations 

Each generation takes time t to occur" 1 This time 
represents both the actual fission process and the neutron's 
travel time from its original nucleus to the one it 
fissions. Since, hovever, it takes less than 10'** seconds 
for the nuclei to fission, t is essentially the neutron's 
travel time betveen nuclei. At the Implosion density, on 
the average a £rse neutron vill travel 2 cm in the fissile 
material at 6% the speed of light. So then t=(2 cm)/(.06x c) 
«1.11 nanoseconds. Thus 55 generations, the entire process, 
takes 55t= 61 ns. 

Note that after 49 generations about 5% of the 3. 75x10* * 
nuclei have been fissioned. So then, 95% of the energy 
generated by the 5 kg of Pu is generated in the last 6 
generations, that is in the last 6. 6 ns. A plot of the 
number of neutrons and the yield as a function of time are 
given in figure 3. 

In a nuclear explosion exceedingly high energies and 
therefore high temperatures (hundreds of millions of degrees 
Centigrade) and high pressures (million of atmospheres) 
build up very rapidly. 

The amount of energy produced in the 61 ns of our model 
explosion is easy to calculate: 

3. 75x10* * fissions x 240 Hev/fission « 9x10" Hev 

* 1.44x10** ergs. 



97 








in rtfcitom 



^jlur 



Figure 3- * plot of the nueber of neutron, end the yield .. 

a function of time. 

At the end of the fission process .11 this energy is 

contained in a volume of W™*^*™ £ B ° th " t ^ 
energy density of the ball of fission debris is. 

(1. 44x10- » ergs)/<50 cm' ) - 2.a8xl0'« erg/cm» 

This energy, E, is divided between the ' .ateri.l of the 
debris and the radiation field, so then 

E « C,T ♦ C r T* 
<C. « 6.2 x 10' erg K" * «nd C, - 7.6x10-- erg K->. 

-i _j*je,n the radiation term dominates, so 
♦ n r n h E° h . C n " 9y ",l"dinr. t.»Per.Jure of T - 25e .illion 

tne sun ««« . , . «ives out photons with *nergles 
te^eratureeen^ object fllve. ^ P ># ^ ch . r . ct . ri . tlo 

ener B Iea of Toft X- end %..-.' r.dl.tlon r.epectively. 

10» • atmospheres. 

2s=r--jrs=^. ^per^z-f r /.. s=a 



l 



98 



degrees Centigrade and pressures of s few thousand 
atmospheres. 

There are therefore four fundaaental differences between a 
fission explosive and conventional explosives. These are: 

{1) the yield per kg of explosive (1 kg of fissionable 
material * 16 kilotonnes of TNT). 

(2) the very different temperatures that the two 
explosions generate ( fission weapons produce temperatures 
5000 times greater than conventional explosives). 

{3> the extremely high pressures produced by a fission 
explosion. 

<4) the production of radioactive fission products and 
high energy neutrons (which can activate non-fissile nuclei) 
in a fission explosion. 

Today, the size, mass and efficiency of fission bombs have 
increased dramatically (see Table 1). 



Type 


Size 


Fission Material 


Yield 


Yield to mass ratio 


Nagasaki 
(1945) 


3 m long 
1.5 m 
wide 


8 kg of Pu-239 


20 kt 


5 000 
Weight: 4 tonnes 


1980 


25 cm 
long 

20 cm 
wide 


5 kg of Pu-239 


40 kt 


1 000 000 
Weight: 50 kg 



Table 1- Comparison of the Nagasaki Weapon 
weapon. 



to 



modern 



The ma 
to I 
togeth 
apart 
of the 
design 
accele 
of man 



ximum explosive yield achieved by implosion is limited 
few tens of kilotonnes by the problem of bringing 
er a critical mass of fertile material before it flies 
because of the energy generated by the explosion. One 
significant, yet simple, advances in fission bomb 
since the Nagasaki type bomb is the imploding 
ration of the tamper in order to multiply by a factor 
y times the intensity of the implosion compression 



99 



■hock wave. A spherical layer of lightweight, rigid, yet 
compressible material between the tamper and the core allows 
the tamper and fission material to be separated to an 
sccurate distance and allows a deadspace for the tamper to 
be accelerated to a high inertia before impacting and 
imploding the fission core. Another significant advance in 
weapon design is the boosted weapon, in which the crude 
initiator (the source of neutrons in the centre Of *»• 
fission core which initiates the neutron production)' of 
the Nagasaki type bomb has been replaced with an electronic 
initiator (a small lightweight source-, siniature particle 
■ccelerator -of high energy neutrons) and a pellet of fusion 
material such as Lithium 6-Deuteride or a gaseous Deuterium- 
Tritium mixture at the centre of the core, which undergoes 
compression, heating and fusion upon implosion and releases 
copious quantities of high energy neutrons. 

Today, using these new techniques, it is P°ff ibl * *° ° btain 
fission explosions of anywhere between 1/2 kt to M kt. A 
schematic of a modern fission weapon is given in figure 4. 




Electronic initiator 
Weapon electronics 

Explosive Lenses 

Weapon casing 



Tamper 



Neutron reflector 



Inertia! dead space 



Booster fuel pellet 



Pu-239 core 



Slow fissioning U-235 shell 



Figure 4- Sche 



matic of a modern fission weapon. 



100 



The Fusion Weapon 

The fusion process is the opposite of the fission process. 
In fusion light nuclei are fused to create heavier nuclei. 
In fusion veapons the heavier isotopes of Hydrogen, 
Deuterium (D> and Tritium (T), are fused together to form 
Helium. The D and T fusion fuel is either present initially 
or are produced, as in a "dry" fusion weapon, in a complex 
reaction involving a special Lithium 6-Deuterlde material. 
The fusion reaction releases energy and Is accompanied by 
the emission of neutrons (see Table 2). 



*H 



!h- 



* 



He 



♦Jn 



Catalyzed DD reaction 
6 ?H— »2jHe + 2JH +?> 

In a "dry" fusion weapon 




12/5% 



+ ?H 

+ JH 

+ !h 

+ JH 
+ jHe 

♦ »h 



+ 
+ 



17.6 Mev 
43.2 Mev 



1 



t 

«n 



+ JH 
+ *» 5 Mev 



+ ~ 5 Mev 
+ *v 5 Mevj 



Exofkerfcilc 



fH 



Em Softer* ?& 






Table 2- List of reactions that occur in a fusion weapon. 



There is no 
therefore in 
yield of (pur 
chain reaction 
chain reaction 
fusion chain 
nuclei are gi 
of electrical 
nuclei. 



critical mass for the fusion process and 
principle there is no limit to the explosive 
e) fusion weapons. However, where a fission 

is easy to start (one neutron will initiate a 

in a critical mass 'of fissile material), a 

reaction is possible only If the reacting 

ven a high enough energy to overcome the force 

repulsion between their positively charged 



In order to initiate the DT reaction a temperature of a 
hundred million degrees Centigrade and high pressures are 
required' * ' . Conditions such as these can be found in a 
fission weapon a split second after Implosion. A fusion 
weapon, therefore, consists of a fission stage, with a 
fission bomb acting as a trigger, and a fusion stage, in 
which the heavy Isotopes of Hydrogen are ignited by the 



-101 



temperatures end pressures produced by the Mission trigger. 
To enhance the explosion even further a third stage is 
normally added, the fissioning of a U-23S cladding. 

Up until 1979 the tanner in which the fusion stage was 
triggered by the first fission stage was the best kept 
military secret in the world'". This secret design is 
known as the Ulam-Teller configuration and was discovered by 
Stanislav Ulam and Edward Teller at Lob Alamos in February 
1951 (see figure 5). 



Fission Stage 
(first fission 
trigger) 



Fusion Stage 




U-238 cladding 
Exotic polystyrene 
Fusion fuel 

Second fission trigger 



Figure 5- The Ulam-Teller configuration 
military secret in the world. 



*— the best kept 



There are three stages to the detonation of a typical fusion 
weapon: fission, fusion and more fission' ••' . The entire 
process of detonation takes a few millionths of a second to 
complete and goes through a number of sequential steps (see 
figures 6 through 11). 



102 



The first stage is m sophisticated, miniaturized fiBsion 
trigger which provides the second stage with the energy 
needed to begin the fusion reaction. 

The second stage is the mechanism which captures the fission 
energy of the first stage and uses it to drive the fusion 
process. The design of this second stage is the Ulam-Teller 
configuration secret. 

The trick to the Ulam-Teller configuration is it allows the 
second stage to finish its task of fusion before the 
expanding fireball of the first stage engulfs the second 
stage and destroys it. About one hundred billionth of a 
second is all the time available before the casing begins 
to fly apart. In the task of transferring the energy 
produced by the first stage to the second the energy of the 
X-and gamma radiation is the only mechanism fast enough and 
manageable enough to be harnessed for that purpose. 

The X- and gamma radiation travel at the speed of light, 
more than a hundred times faster than the expanding debris 
from the exploding fission trigger. If the fission trigger 
and the fusion fuel are separated some distance apart the 
radiant energy from the trigger will have time to race ahead 
of the expanding trigger debris and reach the fusion fuel 
first. 

The cylindrical shape of fusion weapons plays an important 
role in determining how the radiant energy will be 
distributed inside the casing. The first stage is located 
inside one end of a long hollow cylinder casing, and the 
fusion fuel is located inside at .the other end. The 
cylinder is normally large enough to contain the fission 
trigger and leave a few centimetres of free space around the 
inside. 

The cylindrical casing is more than just the package that 
holds the first two stages apart. It is also a radiation 
tamper designed to contain the radiation from the fission 
trigger and initiate the implosion of the fusion fuel 
pencil. The contained X- and gamma radiation is absorbed by 
an exotic, high density polystyrene-type material which 
surrounds the fusion fuel pencil. This material is 
transformed into a highly energized plasma onions (mostly 
electrons produced by the gamma radiation) which ^tivates 
and implodes the fusion fuel pencil (see figures 6 through 
11). 

The fusion fuel pencil is surrounded by a U-238 fusion 
tamper. As the polystyrene activates the fusion tamper 
implodes inwards compressing and heating the fusion fuel. 
In the centre of the fuel pencil a thin diameter rod of 
enriched U-235 or Pu-239 runs its length. This rod of 
fissionable material is compressed to supercritical as the 



103 




Figure 6- Imploeion of the fiaaion trigger. ^J^Ina tne 
the high expioaivea in the fiaaion trigger begins the 
implosion. ?he space between the tamper and the core allova 
the tamper to develop a great deal of inertia before 
impacting the core. 



104 




initiation. 



,. The fission core is 
normal density, going 



Figure 7- Neutron 

compressed to many times its 

supercritical. Neutrons produced by the electronic 

initiator start a explosive chain reaction in the fissile 

material. 



105 




Figure 6- The chain reaction ends. The fusion fuel at the 
centre of the core showers the core vith additional 
neutrons, boosting fission efficiency. As the core expands to 
its original size the chain reaction ends, completing the 
first stage in the detonation. The X- and gamma radiation 
contained within the weapon activates the special 
polystyrene-type material. 



106 




Figure 9- Polystyrene activation. The special polystyrene 
material is transformed into a highly energized plasma of 
ions vhich implodes the fusion fuel pencil, compressing both 
the fusion fuel and the second fission trigger. The second 
fission trigger goes supercritical and begins to heat and 
compress the fusion fuel from the inside. The first fission 
trigger has begun to fly apart. 



107 




Figure 10- Fusion reaction. The fusion fuel and the second 
fission trigger react virtually simultaneously and 
completely throughout the highly compressed and heated fuel 
pencil. The high energy neutrons produced in the fusion 
react begin to fission the U-238 tamper and casing, 
increasing the explosive yield even further. 



108 




Figure 11- The weapon, having completed the three stages of 
f ission/f usion/f lesion, flies apart in a giant fireball, 
emitting high intensity X- and gamma radiation. 



109 



fusion fuel pencil surrounding It implodes. Free neutrons 
start a chain reaction in the rod. It then becomes a second 
fusion trigger (the third stage) which heats the fusion fuel 
from the inside while the activated polystyrene compresses 
from the outside. 

The U-238 tamper then begins to fission as high «nergy 
neutrons produced in the fusion reaction escape the 
imploding fuel pencil. Up to 905C of the total explosive 
yield of a fusion bomb can come from the third stage fission 
reaction, and most of the radioactive fall out as veil. 

The state of the art fusion weapon, the W-80 (the warhead of 
the nuclear-armed cruise missile and Hark 12A missile 
warhead) probably has it yield divided in this manner: 



Stage 


Yield 


Remark 


1st 
2nd 
3rd 


40 let 
180 kt 
130 kt 


Fission trigger 
Fusion 
Fission 



Table 3- Division of yield in the W-80. 



The W-80 has a mass of approximately 125 kg, where about 40- 
50 kg is taken up by the first stage trigger. Thus the 
yield to mass ratio is (350 x 10* kg)/(125 kg) = 2.8x10'. 

A rough approximation of the fission and fusion fuel 
requirements of stages 2 and 3 is easy to arrive at. Since 
1 kt of energy comes from 1.45 x 10« * fissions, we need 130 
x (1.45 x 10* a ) =1.9 xl0* ■ neutrons from the fusion process 
to fission the U-238 tamper. In a fusion reaction 1.53 x 
10* * fusions release 1 kt of energy. Therefore in the 
second stage 180 x (1.53 x 10 84 ) = 2. 8 x 10*' fusion occur. 
Since one neutron is released per fusion reaction til 3, 
fifteen times the number of neutrons are produced during the 
fusion reaction than is needed to completely fission the U- 
238 tamper (the third stage). If there is a total 
fissioning of the third stage, about 7 kg of Pu-239 and U- 



110 



238 would be needed to produce the 130 kt. If ve assume ell 
the fusion fuel is used up In the fusion reaction then the 
amount of fusion fuel required to produce 180 kt of energy 
is; 180 kt x (1.53 x 10* * fusions/kt)« 2. 8x 10** fusions. 
This translates into (2.6 x 10'* fusions)/ (6. 023 x 10* 3 
atoms/mole) * 465 moles. Using Lithium 6-Deuteride as the 
fuel (molar veight 6 gm/mole) means that 3.7 kg of fusion 
fuel vould be required. 

When Lithium 6-Deuteride is used as a source of Tritium for 
the DT reaction a neutron is needed to break up the Lithium 
into a Tritium and a Helium atom £113. This is a slov 
neutron originally produced by a DT reaction and then slowed 
dovn in Lithium or Beryllium to make it suitable for 
interacting with the Li-6. Thus a small amount of 
microencapsulated Tritium and Deuterium must be included in 
the fusion fuel to act as a catalyst for the LiD neutron 
reaction. 

The neutron weapon, also known as the enhanced radiation 
device, does away with the third stage of the fusion weapon 
described above. In the neutron weapon a high density 
material, probably Tungsten alloyed with nickel, iron and 
rhenium, replaces the U-238 in the cladding as well as the 
casing, reducing significantly the fall-out produced. As 
well, the LiD fusion fuel is replaced completely with 
microencapsulated Tritium and Deuterium and no third stage 
fission trigger is used. It is the closest nuclear weapon 
technology has come to date towards the development of a 
pure fusion weapon. The high energy neutrons released in 
the fusion reaction, and not used in any further weapon 
process, are what make the neutron weapon an enhanced 
radiation weapon. 

One result of underground testing over the last fifteen 
years is the development of these special non-fissile 
reflectors that will set off half a kt of fusion explosion 
with as little as half a kt of fission trigger, and with no 
second fission trigger. A drawback to this type of weapon 
is that the Tritium required to make this "clean" weapon 
work has to be produced in special, nuclear reactors and 
Tritium has a short half-life, requiring continual 
replenishment of the DT fuel. 

The newest generation of nuclear weapons consist of the X- 
ray Driven Laser, in development in the USSR and at Lawrence 
Livermore Laboratory in the USA, in which the X-rays from a 
small fusion weapon pumps a special lasing material 
producing a highly directional source of concentrated 
energy. 



Ill 



Conclusion 



This brief introduction should not be considered conclusive. 
Many important details vould need to be explored in greater 
depth before a functioning nuclear weapon could be designed. 
However, the broad concepts behind their workings has been 
presented in sufficient detail to allow the lay person to 
understand the inner Mechanisms of nuclear weapons. 

Today nuclear weapons are Mass produced on assembly lines in 
a manner very similar to the production of consumer 
BppliBnces. They have found their way into »vsry level of 
military planning. while it may not be possible to "put the 
nuclear genie back into its bottle, • it should be possible 
to limit the deployment and integration of nuclear weapons 
into the military infrastructure of nuclear weapon states. 
It should also be possible to limit the proliferation of 
nuclear weapons to new states. 

As Jerome Weiener and others have pointed out, there exist 
no nuclear experts. Sadly lew lay people have taken this 
fact to heart. There has in recent years been a mad 
proliferation of Western Peace and Protest groups who have 
not helped significantly to advance the cause of Nuclear 
Arms Limitation. A general lack ol technical, historical 
and political understanding on their part- has clouded the 
issue with rhetoric and questionable politics. To achieve 
the limitation of nuclear weapons technology is a difficult 
undertaking, requiring well developed technical and 
diplomatic expertise. The reading of the history of any 
recent Arms Control effort would immediately make this fact 
quite clear. Mindful of these facts, the lay person's best 
strategy towards assisting humankind in its effort to 
coexist peacefully and to limit nuclear technology is to 
become better informed individuals and to seek realistic and 
concrete improvements. As Albert Einstein pointed out in a 
letter in 1947; 

" Through the release of atomic energy, our generation has 
brought into the world the most revolutionary force since 
prehistoric man's discovery of fire. This basic power of 
the universe cannot be fitted into the outmoded concept of 
narrow nationalisms. For there is no secret and there is no 
defense; there is no possibility of control except through 
the aroused understanding and insistence of the peoples of 
the world. • 



112 



Footnotes 

[13 A nunber of excellent books exist to provide an 
historic and technical overview of nuclear weapons 
technology. Some excellent reading can be found in; 

The Smyth Report, H. D. Smyth, U. S. Government Printing 
Office , Washington, D. C. 1945 

Brighter Than a Thousand Suns. Robert Jungk, Harcourt Brace 
Jovanovich Inc. , New York, 1958 

Arsenal: Understanding Weapons in the Nuclear Age. K. 
Teipis, Simon and Shuster, New York, 1983 

The Aftermath, (ed. ) J. Peterson, Pantheon Books, New York, 

1983 

The Soviet Union and the Arms Race. D. Holloway, Yale 
University Press, Boston, 1983 

International Arms Control. <ed. ) C. Blaker and G. Duffy, 
Stanford University Press, Stanford, 1984 

Progress in Arms Control. (ed. ) B. Russet t and B. Blair, 
Scientific American Reprints, W. H. Freeman and Co., San 
Francisco, 1985 

[23 When a neutron travels in fissile material along a path 
length known as the mean free path -A- , the probability of 
the collision with and fission of a nuclei is approximately 
70% . Consider the mean free path .^ of a free neutron in 
pure material given by -A- ■ l/( N O" f ), where N is the 
number density of the fissile nuclei and CT f is the fission 
cross section. For Pu-239 N « 5 x 10* • cm" " and <5~ , * 2 
x 10-** cm« giving -A. « 10 cm.. 

[3] Some sources for critical mass data include the book 
Aftermath mentioned above, as well as the following two 
sources; 

»» 
Taylor, Annual Review of Nuclear Science, Vol. 25, 1975 p 

412 

A. de Volpi, Trans. American Nuclear Society. Vol. 30, 1978 p 
298 

B. Lovins, Nature. Vol. 283, 1980 p 817. 

[43 The technique of Nontecarlo simulation was developed by 
the American mathematician Stanislaw Ulam at Los Alamos in 
the late 1940s. It involves the use of a random number 
generator to create input (numbers) for a mathematical 



113 



function which in turn provide. • simulation of » system 
process such as a nuclear chain reaction. 

153 Nuclear physics is a far wore complex subject than is 
outlined in this paper. For the sake of clarity certain 
assumptions have been made to simplify the description of 
the inner mechanisms of nuclear weapons. A more in depth 
study of the physics of nuclear weapons would require 
considerable expansion to the material presented in this 
paper. 

[63 If n neutrons are present at time t, their number will 
increase in the time dt by dn - f(t)n(t) dt. Here f(t) is a 
complicated function of geometry, density and time. 
Typically, f(t) is initially of the order 10« «' » . As long 
as f (t) can be considered a constant, the neutron number 
will increase exponentially so that n(t) - n<0) expCt/t, ) 
where t,=i/(f<t)) is called the generation time. 

£73 After the implosion has proceeded an optimum length of 
time a source or generator of neutrons initiates a flood of 
free neutrons at the centre of the fissile material that 
starts the explosive exponential growth in the neutron 
population. In the Nagasaki bomb it was a_ Beryllium- 
Polonium sphere which when compressed released a shower of 
low energy neutrons. In modern weapons the initiator is a 
miniature linear accelerator that uses the DT reaction to 
create copious numbers of free neutrons which flood the 
imploded core and start the explosive chain reaction. 

[83 The fuel temperature required for the DT reaction is on 
the order of 100 million degrees Centigrade. The 
confinement requirements is given by the Lawson criterion 

(plasma ion density )x (confinement time) > IV. «r . 

[93 For a description as to how the Ulam-Teller 

configuration became public knowledge see A. De Volpi et al. 

Born Secret: The H-Bomb. th » Progressive Case, and National 
Security. Pergamon Policy Studies, 1981 

or/ Progressive. Nov. & Dec. 1979 

[10 3 The neutron weapon has a slightly different 
configuration to that of a typical fusion weapon < see below 
in article). 

[113 See table 2